The present invention relates to a converter circuit having at least one switching device and a diode arranged to be conducting when the device is turned off, and reverse biased when the device is turned on, as well as circuitry and a module according to the independent claims herein.
The invention is particularly, but not exclusively, concerned with the problems arising in converter circuits used for high voltage applications, and it is directed to all types of such converter circuits, such as inverters converting a direct-current voltage to an alternating voltage, rectifiers converting an alternating voltage into a direct-current voltage, and direct current converters converting a direct-current voltage into a higher or lower direct-current voltage.
Such converter circuits are connected to a load, for instance an electric motor, which is supplied with a certain voltage obtained by the converter circuit. A diode in this circuit is used as a so-called freewheeling diode to improve the capability of the converter circuit to produce a voltage with characteristics aimed at primarily smoothing out abrupt changes in the circuit due to switchings of the switching device, and to protect the latter by taking care of the conduction of the current which the load calls for when the switching device is turned off.
Prior art diodes in such converter circuits, and mostly made of Si, have some disadvantages which cause severe problems. In fact, these diodes are the devices which tend to limit the performance of the entire circuit, mainly because of the inherent limitations in switching performance with large switching losses and risk of destruction due to dynamic avalanche. At higher voltages, diffused pn-junction diodes, having high switching losses, are used.
The main problem arises when the switching device of the circuit is turned on, and the diode becomes reverse biased. The diodes of this type have a relatively high reverse recovery charge, i.e. a high amount of excess electrons and holes are stored within the n- and p-regions of the device, and this charge has to be withdrawn when the diode is reverse since a reverse current, appreciably higher than the saturation or leakage current in the turned-off state of the diode, will continue to pass therethrough for some time. This results in high switching losses and heat dissipation in the diode, since the voltage over the diode during this reverse current will be high after the turning on of the switching device and also the turning off of the diode. Also, this reverse current through the diode causes extra turn-on losses within the switching device. Furthermore, the diodes of Si become unstable even at comparatively low temperatures.
The quality of the current obtained by such a converter circuit will increase with increasing frequency of the switching of the switching device, but the switching losses also increase with increased switching frequency, so that the frequency of the switching has to be kept at such a low level in order that the devices of the converter circuit will not be destroyed due to overheating.
Furthermore, the diode may be destroyed due to dynamic avalanche when there are excess charge carriers left in the diode when the reverse blocking voltage over the diode has become high. Thus, there usually is a need to reduce the turn-on speed of the switching device, which is not desirable for optimal performance of the circuit.
There is a further problem, although mostly not that accentuated, when the switching device is turned off. The diode will then be turned on with a very fast increasing current. The large Si diodes used as high-voltage power devices will, namely directly after turning on of the diode before the creation of a required amount of excess charge carriers therein, have a high resistance resulting in a high voltage, a phenomena called forward recovery. This voltage overshoot can have negative effects on the power electronic circuit, creating unwanted losses. It can also stress the diode and limit the lifetime of the diode thereby influencing the reliability of the entire converter circuit. This also causes voltage overshoots for the switching device. When the diode, is used as a snubber diode this forward recovery overshoot is often the limiting factor for the switching devices, especially if it is a GTO.